The Factors Affecting In Planta Spectra of Screening Pigments and Radiation Screening Efficiency

To understand the functioning of optical screening of solar radiation in living plants, it is important to realize that the in vivo absorption properties of pigments differ considerably from those of isolated pigments in vitro (Naqvi et al. 1997). Flattening of absorption bands and bathochromic shifts of the maxima are evident in vivo in comparison with absorption spectra of pigments recorded in organic solutions (Fukshansky 1981; Fukshansky et al. 1993; Gitelson et al. 2003b, 2006).

The spectrum shape and the positions of the maxima in pigment spectra are also influenced by the environment (polarity and chemical composition) of the pigment molecules, the organization of the structures containing the pigments, and other effects, such as packaging (Berner et al. 1989; Britton 1995; Gonnet 1999; Merzlyak et al. 2009; Naqvi et al. 1997, 2004; Smith and Markham 1998). Spectral properties of photosynthetic pigments (chlorophylls and carotenoids) are also affected in vivo by proteins which bind them to form pigment-protein complexes of the photosynthetic apparatus (Britton 1995; Green and Durnford 1996).

The spectra of vacuole-contained pigments can be changed significantly as a result of intra- and intermolecular copigmentation, the formation of various complexes, and the effects of pH, metal ion chelation, tautomerism, etc. (Asen et al. 1972; Lancaster et al. 1994; Smith and Markham 1998). These effects lead to a significant hyperchromic effect and profound bathochromic shifts in the absorption spectra of vacuolar contents.

The in planta spectra of screening pigments are also greatly influenced by scattering (multiple internal reflection/refraction) which arises from the complex morphology of plant cell tissues and causes a considerable increase in the effective path length of solar radiation absorption within plant tissues (Butler and Norris 1960; Fukshansky 1981; Vogelmann 1993). As a result, the same amount of a pigment in planta absorbs several times more strongly than in an organic extract (Butler and Norris 1960).

The above-mentioned mechanisms and processes can significantly influence the efficiency of radiation screening in certain wavebands. For example, the long-wavelength absorption maximum of flavonols in vitro is situated in the UV-A region (Markham 1989), but in planta the absorption band is flatter than in solution and its maximum can be shifted 20-30 nm toward longer wavelengths (Smith and Markham 1998). As a result, flavonols obviously absorbing in the UV range (the maximum is at 350-360 nm) begin to exert detectable screening in the broad band from the violet to the blue-green region of the spectrum (Havaux and Kloppstech 2001; Merzlyak et al. 2005b). This is especially significant for protection against

UV-A radiation, which is not so harmful as UV-B radiation but reaches Earth's surface in much higher fluxes in comparison with UV-B radiation (Bjorn and Murphy 1985). Mechanisms similar to those outlined above also influence the in vivo spectra of anthocyanins. As a result, these pigments can gain the ability to intercept visible radiation from the blue-green to the orange part of the spectrum (Merzlyak et al. 2008a, b). This could be of special importance for the protection of senescing leaves and ripening fruit.

The concentration-dependent broadening of screening pigment absorption bands often causes even more profound bathochromic shifts of the long-wavelength absorption slope of phenolic and carotenoid in vivo absorption, enhancing the ability of phenolics and carotenoids to intercept radiation at longer wavelengths, where they possess low absorption coefficients (Markham 1989; Strack and Wray 1989). This effect could be additionally enhanced owing to the lengthening of the absorption optical path due to light scattering (see above).

Collectively, the considerations presented above suggest that the effects influencing the pigment spectra in planta should be taken into account for the correct estimation of real photoprotective capacity of the screening pigments.

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